Spectrometers are well known devices for measuring the spectra of laser beams (i.e., the intensity of light in the beam as a function of wavelength). Ultraviolet laser light sources used for modem integrated circuit lithography are required to have very narrow bandwidth and operate within tight bandwidth specifications. Spectrometers used to measure the spectra of these lasers can be divided into two main categories: diffraction grating based spectrometers and Fabri-Perot etalon based spectrometers.
A description of a typical KrF excimer laser used for lithography is provided in U.S. Pat. No. 5,991,324 which is incorporated herein by reference. There are two spectral bandwidth characteristics of these lasers which are referred to extensively in microlithography applications. These are the spectral bandwidth of the laser measured at 50 percent of the peak intensity, called its full width at half maximum bandwidth (abbreviated FWHM), and the spectral bandwidth, which contains 95% of laser energy called the 95% integral bandwidth (abbreviated 95% I). It is very important that the laser is always operating within specifications during microlithography chip manufacturing because spectral broadening causes blurring of the integrated circuit features being printed on silicon wafers which would result in yield problems. Therefore, it is very important to provide continuous accurate monitoring capabilities for the laser spectrum.
A measure of the quality of a spectrometer is its slit function. This is the spectrum which is recorded by the spectrometer when measuring a very very narrow spectrum. For a spectrometer to accurately measure the spectrum of a laser beam, the slit function bandwidth of the spectrometer itself should be substantially smaller than the laser bandwidth.
The nominal wavelength of a KrF laser is in the range of about 248 nm and the nominal wavelength of an ArF laser is in the range of about 193 nm. Current KrF and ArF lithography lasers operate at very narrow bandwidths within these ranges; with FWHM bandwidths of about 0.4 pm to 0.6 pm and with 95% I bandwidths of about 1.5 pm. A very good grating spectrometer such as the ELIAS model Echelle spectrometer supplied by Lasertechnik Berlin has a slit function with FWHM bandwidth in the range of about 0.14 pm and an 95% I bandwidth in the range of about 0.54 pm. (This slit function was measured using as a very very narrow spectrum a frequency doubled beam of an argon ion laser at a normal wavelength of about 248.25. The FWHM bandwidth of the frequency doubled line is about 0.04 pm.) Obviously, the slit function values of this spectrometer are somewhat smaller but not substantially smaller than the bandwidths being measured. The result is that the measured laser spectrum is not a true spectrum of the laser beam but a convolution of the laser beam spectrum and the slit function spectrum of the spectrometer. Deconvolution techniques are available in the prior art which utilize Fourier transforms to deconvolve the measured spectrum; however, available formal deconvolution algorithms are difficult to use and often lead to poor results especially for the 95% I values because the data at the outer wings of the spectra are subject to wide statistical variations.
What is needed is a spectrometer, capable of producing good FWHM and 95% I data and a simple, accurate, and easy to use method for using these measured data to calculate accurate, consistent and reliable bandwidth data.
The present invention provides a simple, reliable, easy to use method for calculating bandwidth data of very narrow band laser beams based on bandwidth data obtained with a spectrometer in circumstances where the laser bandwidths are not large compared to the slit function of the spectrometer.
The slit function of the spectrometer is determined. Spectral data of the laser beam are measured with the spectrometer to produce a measured laser beam spectrum which represents a convolution of the laser beam spectrum and the spectrometer slit function. This measured laser spectrum is then mathematically convolved with the slit function of the spectrometer to produce a doubly convolved spectrum. Bandwidth values representing true laser bandwidths are determined from measured laser spectrum and the doubly convolved spectrum.
Preferably the true laser bandwidths are calculated by determining the difference between xe2x80x9ctwice a measured laser bandwidthxe2x80x9d and a corresponding xe2x80x9cdoubly convolved bandwidthxe2x80x9d. This method provides an excellent estimate of the true laser bandwidth because xe2x80x9ctwice the measured laser bandwidthxe2x80x9d represents two laser bandwidths and two spectrometer slit function bandwidths and the xe2x80x9cdoubly convolved bandwidthxe2x80x9d represents one laser bandwidth and two spectrometer slit function bandwidths. Thus, the difference is a representation of the true laser bandwidth.
In a preferred embodiment the bandwidth parameters measured are the full width half-maximum bandwidth and the 95% integral bandwidth.